I. Technical Field
The present invention relates to a position sensor for precise positioning which is particularly suitable for devices which are required to be precise and miniaturized, such as portable imaging devices, medical devices, and the like, since the position sensor can be made small with non-contact detection.
II. Description of the Related Art
In recent years, imaging devices, such as cellular phones with an image capturing function, digital cameras, and the like, have become small and precise with more functions. Such imaging devices include mechanisms to move lenses and/or other components for autofocusing, zooming, image stabilizaion, and the like.
In the field of optical discs, optical pickups compatible with high-density recording include lens moving means for compensating for aberration of systems. For applying the devices to portable use, or incorporating to portable game machines, or the like, the optical pickups are required to be miniaturized.
For moving lenses with a high speed and high accuracy, it is desirable to provide a position sensor for a moving lens or the like in order to improve the response of a control system. However, motors and/or actuators which can be used in such small devices are limited in terms of shape and/or electric power. Thus, non-contact sensors that are desirable in view of the load, and optical detection, magnetic field detection, and the like, are used for sensor means.
A position detection means includes: digital means which mainly performs a pulse count of the amount of shift from an initial position; and analog means which mainly specifies an absolute position. As is well known, the former is accurate but requires an initial position detection operation, and the latter can specify the position instantly but the properties vary depending upon a temperature, and/or S/N ratio, and thus, the accuracy is lower than that of the former. These characteristics are complementary.
Small portable devices are often required to have immediacy. Thus, analog means may be useful in many cases if the accuracy is permissible.
When a position sensor is formed as such analog means, a position sensor using a magnetic field is employed in many cases, since optical detection has a nonlinear complicated temperature characteristic and is relatively expensive.
A conventional example of a position sensor using a magnetic field will be described based on Japanese Patent Gazette No. 3597733 (FIGS. 2 and 3). In FIG. 13, reference numeral 91 denotes a Hall element which serves as magnetic field detection portion. The Hall element 91 generates a voltage proportional to a magnetic flux density in the Z axis direction shown in the figure.
Reference numeral 92 denotes a permanent magnet which serves as a magnetic field generation portion. The permanent magnet 92 is polarized (magnetized) in a direction indicated by arrow M in the figure. The permanent magnet 92 is formed such that both ends become distant from the Hall element 91.
The permanent magnet 92 is formed so as to be movable relative to the Hall element 91 in X axis direction shown in FIG. 13.
When the permanent magnet 92 moves relative to the Hall element 91 in the X axis direction shown in FIG. 13, a magnetic field in the Z axis direction being applied to the Hall element 91 is changed between the N-pole and the S-pole, and a electric signal corresponding to the change in the magnetic field is output from the Hall element 91. This means that the position of the permanent magnet 92 in the X axis direction is detected from the output of the Hall element 91.
If the relationship between the position of the permanent magnet 92 in the X axis direction and the output voltage of the Hall element 91 is linear, that is, if the position of the permanent magnet 92 in the X axis direction and a strength of the magnetic field in the Z axis direction being applied at the position of the Hall element 91 are linear, conversion from the output voltage into the position can be readily performed.
With an ordinary magnet, a magnetic field is strong at magnetic poles in both ends, and linearity deteriorates. Thus, in this example, the permanent magnet 92 is formed to have a polygonal shape such that both ends in the magnetizing direction become distant from the Hall element 91 for compensation for improving the linearity.
Japanese Patent Gazette No. 3597733 further describes an example of improving the linearity by forming the permanent magnet into a curved surface.